If they don't care, then it shouldn't matter that third-party options exist. The same thing happens on Android, nobody uses F-Droid even if the apps are better and cheaper. The fact that it exists puts meaningful pressure on competitors though, and fills the niche that I use the device for.

The "walled Kindle/Nook garden" isn't similar Apple's ecosystem. A Kindle or Nook will let you access it's EMMC and put Epub files or PDFs wherever you need to. It functions indistinguishably from a store-bought file and doesn't even make you enable Developer Mode to get there. It's "out of reach" in the sense that you need the skills to follow a Wikihow article to do it - my grandma could figure it out.

iPhones don't let you install IPA files on equal-footing as Apple, ever. That's the problem, and it's what the DMA remediates. I don't care how you or the normies feel about it any more than I consider the public sentiment towards Bell telephone or Internet Explorer.